Once-through steam generator with means to provide saturated feed water



Dec. 5, 1967 R. K. SAYRE ONCETHROUGH STEAM GENERATOR WITH MEANS TOPROVIDE SATURATED FEED WATER Filed Dec. 24, 1964 M WAGE-T United StatesPatent O 3,356,135 ONCE-THROUGH STEAM GENERATOR WITH MEANS To PROVIDESATURATED FEED WATER The present invention relates to an improvedoncethrough steam generator and more particularly to a oncethrough steamgenerator having the design advantage of eliminating thermal stresses,boiling stability, and flow distribution difficulties between boilingchannels located in parallel.

Several diiculties arise in the employment of the oncethrough steamgenerator, for example, as a component of a nuclear power plant. In suchsteam generators there is a normal operating thermal stress problemwhich is more severe then that of the recirculating boiler because thereis no mixing of the cold feed water with the saturated recirculatingwater. The result of this is that the cold feed water in a once-throughsteam generator will normally come in direct contact with the steamgenerator shell and/or tube sheets, as a result there occur largethermal stresses in such shell or tube sheets.

There is also a problem of insuring boiling stability with once-throughsteam generators. Unless special design provisions are made, boilingchannels with high heat fluxes land high vaporization fractions willoscillate when fed with subcooled water. In the once-through steamgenerator, of course, there is usually employed a maximum degree of feedsubcooling (the feed enters the channel directly without mixing withrecirculating water), and the vaporization fraction is 1.0. Hence,once-through steam generators are highly susceptible to oscillations.

Another difliculty in the design of once-through steam generators isbrought about by what is called boiling disease. It has been shown thatthe pressure drop through the parallel boiling channels with subcooledfeed is virtually independent of mass velocity over wide flow ranges.The result of this is that special design provisions must be made toequalize the flow through any parallel How multichannel boiling pathwaywith subcooled feed.

In carrying out the present invention, thermal stresses in pressureboundary members are minimized by avoiding direct Contact between thecold feed water and the pressure boundary members except the heattransfer tubing. Heat transfer tubing is best able to withstand thethermal stresses caused by this contact with the cold feed water becauseit contains a minimum of discontinuities, restraint, and bendingstresses. Contact between the tube sheet, or shell, and feed water isprevented by leading the feed water directly into a small chamber ormanifold in the tube bundle bounded by tightly fitting bales at the twoends of the chamber and a shell around the bundle covering the sides ofthe chamber. The novel feed heating arrangement according to the presentinvention also insures boiling stability. Generally, boiling systems fedwith saturated water will not oscillate, nor will boiling systems wit-ha very low fraction of vaporization oscillate. According to theinvention, the advantageous result of decreasing or eliminatingoscillations is provided by splitting the boiler into two dynamicallyindependent systems, the rst of which brings the feed water nearly tosaturation or slightly beyond saturation, and the second of whichcarries the output of the rst to dryness. Thus both are independentlystable.

More particularly, the method of splitting the preheating section fromthe boiling section according to the present invention is to make theinertia of the preheating section very high compared with that of theboiling section. The feed water heating flow is confined to annuli ofsmall cross sectional area so that the feed now attains a high velocityand hence a high inertia. Means are also provided to couple the feedwater heating section directly to the entire feed water system by theagency of a completely filled feed Water inlet manifold which containsthe feed water.

The feed heating annuli are made of sufficient length to bring the feedapproximately to saturation. The saturated feed is then brought to acondition of lower velocity in a baiiled boiling section. With theresulting large drop in velocity there will be a corresponding drop ininertia, thereby decoupling the two sections of the boiler.

vThe third problem of flow distribution is solved in two ways. First theboiling section is fed with saturated water. Parallel channels fed withsaturated water do not suffer from flow distribution problems. Secondthe entire boiling section is in the form of a single channel so thatthe only problem is one of water distribution within the channel, aproblem of mechanical mixing rather than a problem of basic flowcharacteristics.

It is accordingly, among the `objects of the present invention toprovide:

An improved steam generator for high power plants having means of anovel design of reducing oscillations in the steam generator.

A once-through steam generator of improved design having means forreducing thermal stresses, and for enhancing boiling stability in flowdistribution between boiling channels.

A once-through steam generator of improved design in which there isprovided means for splitting the boiler portion of the system into twoindependent systems each of which is independently stable, therebyreducing oscillation.

A once-through steam generator of novel design having improved means forcoupling the feed water heating section directly to the entire feedwater system inertially.

A feature of the present invention is the provision of an improvedmanner of llow distribution. Means arev provided so that parallelchannels do not have discontinuous flow distribution and so that theentire boiling section is formed of a single channel in which waterdistribution may be carried out within the channel by mechanical mixing.

The -foregoing objects as well as other Objects, features and advantagesof the present invention will be better understood by referring to thefollowing description and the accompanying drawings in which:

FIG. 1 is a modied side view of a once-through steam generator according-to one version of the invention; and

FIG. 2 is a view in section of a portion of the details of the feedheating section of the steam generator according to the invention.

Referring to FIG. 1, the steam generator depicted therein has an outerboiler wall or shroud 11 surrounding the units of the generator, at oneend of which is a header section 13 having an inlet 15 and an outlet 17for the purpose of providing heating lluid for the unit. The heatingfluid is passed through the generator by means of a tube bundle 19consisting of a plurality of spaced parallel tubes 21 forming `a U witheach end of the U fluidly coupled to the header. The generator has apreheating section 22 and a boiling section 23 through which the tubebundle 19 passes. The steam output is provided at an outlet 24 which maybe located conveniently at the top of the unit.

Referring to FIGS. 1 and 2 together, the feed water from which the steamis to be generated is passed under suitable pressure into the preheatingsection 22 via an 3S inlet 25. The preheating -section has a chamber 27which is defined by walls 29, 31, 33 and 34. The walls 29, 31, 33 and 34are preferably constructed as shown so that they are isolated fro-m theshroud 11. The walls of the chamber are held in place essentially bybeing affixed in any suitably manner to the tubes 21.

The bundle of tubes 19 passes through the chamber 27 so that the feedwater comes in contact with each of the spaced tubes of the bundle. Foreach of the tubes 21 passing through the chamber 27 the-re -is provideda sleeve 3S surrounding each tube. The sleeve is of larger internaldiameter than the external diameter of the tube. Thus each of thesleeves and its associated tube (together) define an elongated annularspace 37 therebetween. Each of the sleeves 35 extends from a positionwithin the chamber 27 continuously along the tube to a suitablepredetermined position in the boiling section 23 downstream of thechamber 27.

The steam generator unit has a longitudinal baffle 39 located centrallyof the unit connected to the end wall 29 but otherwise spaced from thewalls of the chamber 27. The baffle 39 separates one leg of the U-shapedtube bundle 19 from the other leg of the U-shaped tube bundle.

In the boiling section 23 the longitudinal baffle 39 has extendingoutwardly therefrom a plurality of spaced transverse baffles 41. Thereis also a plurality of spaced baflles extending inwardly from the shroud11 and staggered in relation to the baffles 41. The groups of baffles 41and 43 provide in the boiling section a zig-Zag path for enhancingmechanical mixing of the feed water after it passes from the preheatingsection through the boiling section.

At the portion of the baffle adjacent to header unit 13 there isprovided :an aperture 45 for passing the steam into a chamber 47. Thechamber 47 bounds the wall 29 of chamber 27. Steam entering chamber 47passes into contact with the wall 29 of the chamber 27 to enhanceheating of the feed water. A vent 49 for expelling steam is provided inthe chamber 47 for insuring steam flow.

The steam generating unit operates as follows:

When the feed water is passed through the inlet 25 into the chamber 27,the annular -spaces 37 defined by the sleeves 35 adjacent to respectivetubes 21 provide a plurality of discrete high area-to-volume heattransfer regions through which the feed water passes. As the feed waterpasses through the annular spaces 37 it comes into close confinedcontact with each of the tubes 21 so that the feed water is preheatedrapidly to a point at or near saturation or even slightly abovesaturation. Since the feed water in the annuli is heated while confinedthe feed achieves a high velocity and therefore substantially increasesits inertia.

The essentially saturated feed is released at the egress end of theannuli 37 to the boiling section in which the feed water is mechanicallymixed as it passes freely among the tubes 21 in a zig-zag path caused bythe groups of baffles 41 and 43. As the feed water leaves the annuli 37there is a considerable drop in velocity of the saturated feed Water dueto expansion, The inertia of the feed water also drops, the effect ofwhich is to decouple the cooler preheating section from the warmerboiler section of the unit.

In that the boiling section is in effect fed with essentially saturatedwater, and since the saturated feed in that portion of the unit isflowing essentially in a single mixing channel, the water distributionwithin the channel is determined primarily by its mechanical mixing.

Of course, the heating fluid in the tubes 21 serves to heat toa desireddegree of dryness the feed water passing among the ballles 41 and 43 andamong the tubes 21 to the steam Ioutlet 24. Some of the steam will alsopass through the aperture 45 into a chamber adjacent the wall 29 of themanifold. Flow of the steam via the aperture 45 is further enhanced by aprovision of a vent 49 which communicates with the chamber 47. Thissteam adjacent the wall 29 serves to further heat or preheat the feedwater as it is introduced into the chamber 27.

Thermal stresses in the shroud 11 and baffle 39 are minimized becausethe cool walls of chamber 27 are essentially isolated from contacttherewith. Thermal stresses will occur to some extent in the tubing, butthe tubing is normally capable of withstanding such stresses.

Since the action of the feed water passing through the annuli 37 whilebeing heated produces high velocity, high inertia, saturated feed andfurther, since the high inertia feed is then brought to a much lowerinertia by passing the steam from the annuli 37 into the larger singlemixing channel indispersed with the tubes 21, the preheat'ing andboiling system are essentially decoupled thereby preventing oscillation.

The present arrangement is in direct contrast to previous units such asthose having channels with high heat fluxes and 'nigh vaporizationfractions which will oscillate when fed with subcooied water. This isespecially true in once-through generators because of their use of amaximum amount of feed subcooling. ft is yet further appreciated thatsince there is a single mixing channel for the saturated feed, there isno difficulty or necessity for equalizing the flow through any ofparallel flow type channel boiling pathways as in the previous knowndesigns.

It is understood that the present generator may bc cmployed for otherfeed uids besides water. Obviously many modifications and variations ofthe present invention are possible in the light of the above teachings.lt is therefore to be understood that within the scope of the appendedclaims the invention may be practiced otherwise than as specificallydescribed.

What is claimed is.

1. A once-through heat exchanger comprising:

means for providing feed heating fluid for the exchanger;

enclosed chamber means, having Walls, for receiving feed to be convertedinto fluid of a higher energy state;

means defining a plurality of discrete spaced confined paths passingthrough and beyond said chamber;

said last named means having one end connected to receive the heatingfluid and it's other end connected to discharge the heating fluid;

means surrounding each of said confined path means for defining at leastone space therebetween of high area-to-volume ratio, cach of said lastnamed means passing through one wall of said chamber;

channel defining means arranged for providing a mechanical mixing pathfor feed fluid in proximity t0 said confined path means for an extendeddistance in heat transferring contact with said confined path means;

outlet means for said unit, whereby when feed is introduced to saidchamber under pressure, said feed enters the high area-to-volume spaceand is pre-heated therein, said feed being heated to approximately itssaturation, said feed thereby obtaining high velocity and high inertia,said feed when saturated passing near the outlet of said space into asingle mechanical mixing channel in close proximity t'o said confinedpath defining means whereby said saturated feed is brought toconsiderably lower inertia and velocity, said saturated feed thereafterbeing brought into its desired state of dryness by flowing in contact insaid path with said path defining means and out of the system via saidoutlet.

2. A once-through steam generator having a feed inlet and a steam outletcomprising:

a shroud for the generator;

a tube bundle enclosed by the shroud and fluidly coupled at one end toreceive heating fluid and fluidly coupled at its other end to dischargeheating fluid, said tube bundle being the form of a U;

means for receiving the feed water, said means comprising a chamberhaving walls mounted isolatedly from the shroud; I

said tubes passing through said chamber;

a Sleeve for each tube of said tube bundle, one end of said sleeveextending through a wall of said chamber, the inside diameter of each ofsaid sleeves being greater than the outside diameter of each tube ofsaid tube bundle;

a central baille separating the legs of the U of said tube bundle;

said central baille having extending outward therefrom plurality ofspaced transversed balles extending among said tube bundles;

said shroud having a plurality of spaced baille members extendinginwardly towards said central baille.

3. A steam generator according to claim 2 wherein said central baillehas an aperture therein;

said aperture defining the entrance to a chamber located adjacent atleast one wall of said feed water chamber;

said chamber having a vent therein, whereby steam produced in saidgenerator ilows thru said chamber for further preheating of said feedwater.

4. A once-through steam generator having a feed inlet and a steam outletcomprising:

a shroud;

header means for providing heating iluid located at one end of saidshroud;

a U-shaped tube bundle of spaced tubes located in said shroud andiluidly coupled at each end of the leg of said U to said header meansfor passing the heating fluid through said tubes;

baille means located centrally of said generator and dividing the legsof said U to form with the shroud a single channel enclosing said tubebundle;

means located within said shroud for receiving feed water, said meanscomprising a chamber having walls physically isolated from said shroudand from said baille, the walls of said chamber being arranged forsupport essentially by the tubes of said bundle;

means extending into said chamber and deilning an annular space aroundeach tube of said bundle, said means extending Ialong each of said tubesoutward of said chamber for a predetermined distance;

each of said last named means having an outlet for passing heated feedwater into the channel defined by said central baille and said shroud;

whereby upon introduction of feed water into said chamber and into saidannular spaces, said feed water is heated to essentially its saturation,said essentially saturated feed then being passed into said channel formechanical mixing therein in the presence of said tube bundle, wherebysaid feed is brought to its desired degree of dryness.

5. A Once-through steam generator 'according to claim 3 wherein saidgenerator further comprises transverse baille means located in saidchannel. p

6. A once-through steam generator according to claim 4 but furthercharacterized by said central baille having aperture means therein,means defining a chamber iluidly coupled to said aperture means andlocated immediately adjacent said feed water chamber, said immediatelyadjacent chamber having a vent means therein whereby steam generated insaid system may pass through said aperture into said adjacent chamberfor additionally heating the feed water.

7. The once-through steam generator according to claim 5 wherein thetubes of said tube bundle are essentially parallel and spaced from eachother.

8. A once-through steam generator having a feed inlet and steam outletcomprising:

a U-shaped tube 4bundle of spaced tubes;

a shroud surrounding said U-shaped tube bundle;

a central baille separating the legs of the U-shaped bundle;

a header section located at one end of said shroud and iluidly coupledto each leg of said U-shaped tube lbundle for supplying heating fluidfor said tube bundle;

chamber means for receiving feed water, said chamber means beingessentially thermally isolated from said shroud and from said centralbaille and essentially attached to said tube bundle;

means deilning an annular space for each tube of said tube bundle andextending into said chamber means and without said chamber means forpredetermined distance along said tube bundle;

said annular space being of relatively small radial dimension incomparison with the diameter of said tube and thereby providing a spaceof large area-tovolume ratio through which said feed water may pass forheating said feed water to a high temperature and for providing highvelocity inertia for said feed water as it becomes saturated in saidannular space.

References Cited UNITED STATES PATENTS 178,244 6/1876 Koll 122-409620,994 3/1899 Teste 122-242 653,3 72 7/ 1900 Altmann 122-242 673,9085/1901 Jonsson 122-242 X 827,479 7/1906 Towne 165-159 FOREIGN PATENTS741,988 12/ 1955 Great Britain.

ROBERT A. OLEARY, Primary Examiner. MEYER PERDN, Examiner. A. W. DAVIS,Assistant Examiner,

1. AT ONCE-THROUGH HEAT EXCHANGER COMPRISING: MEANS FOR PROVIDING FEEDHEATING FLUID FOR THE EXCHANGER; ENCLOSED CHAMBER MEANS, HAVING WALL,FOR RECEIVING FEED TO BE CONVERTED INTO FLUID OF A HIGHER ENERGY STATE;MEANS DEFINING A PLURALITY OF DISCRETE SPACED CONFINED PATHS PASSINGTHROUGH AND BEYOND SAID CHAMBER; SAID LAST NAMED MEANS HAVING ONE ENDCONNECTED TO RECEIVE THE HEATING FLUID AND ITS OTHER END CONNECTED TODISCHARGE THE HEATING FLUID; MEANS SURROUNDING EACH OF SAID CONFINEDPATH MEANS FOR DEFINING AT LEAST ONE SPACE THEREBETWEEN OF HIGHAREA-TO-VOLUME RATIO, EACH OF SAID LAST NAMED MEANS PASSING THROUGH ONEWALL OF SAID CHAMBER; CHANNEL DEFINING MEANS ARRANGED FOR PROVIDING AMECHANICAL MIXING PATH FOR FEED FLUID IN PROXIMITY TO SAID CONFINED PATHMEANS FOR AN EXTENDED DISTANCE IN HEAT TRANSFERRING CONTACT WITH SAIDCONFINED PATH MEANS; OUTLETS MEANS FOR SAID UNIT, WHEREBY WHEN FEED ISINTRODUCED TO SAID CHAMBER UNDER PRESSURE, SAID FEED ENTERE THE HIGHAREA-TO-VOLUME SPACE AND IS PRE-HEATED THEREIN, SAID FEED BEING HEATEDTO APPROXIMATELY ITS SATURATION, SAID FEED THEREBY OBTAINING HIGHVELOCITY AND HIGH INERTIA, SAID FEED WHEN SATURATED PASSING NEAR THEOUTLET OF SAID SPACE INTO A SINGLE MECHANICAL MIXING CHANNEL IN CLOSEPROXIMITY TO SAID CONFINED PATH DEFINING MEANS WHEREBY SAID SATURATEDFEED IS BROUGHT TO CONSIDERABLY LOWER INERTIA AND VELOCITY, SAIDSATURATED FEED THEREAFTER BEING BROUGHT INTO ITS DESIRED STATE OFDRYNESS BY FLOWING IN CONTACT IN SAID PATH WITH SAID PATH DEFINING MEANSAND OUT OF THE SYSTEM VIA SAID OUTLET.